1. Field of Invention
This invention relates to an illumination system for microlithographically forming patterns on semiconductor wafers. More particularly, it pertains to a lamp housing or shield and to a cooling and ventilating system associated therewith, the lamp serving as the source in the illumination system.
2. Background Information
The type of apparatus for microlithographically forming patterns on semiconductor wafers in which the present invention serves its particular function can be appreciated by reference to U.S. Pat. No. 4,425,037. That patent, which is assigned to the assignee of its present invention, discloses an improved system including a stepper for projection of pattern images onto a predetermined focal plane. Such apparatus includes as a source of illumination a mercury vapor arc lamp.
A fundamental problem that exists in the described context is that one must provide an optimum operating environment for the mercury vapor lamp in an enclosure or housing which shields the area around the stepper from hazardous ultra violet radiation, which is dangerous to people and can damage photosensitive materials used near the stepper.
Moreover, a heat source can destabilize the metrology of the stepper so heat transfer to the rest of the stepper must be minimized.
Another problem that occurs when mercury vapor lamps are utilized is that the mercury, being poisonous, can produce additional hazardous conditions for personnel in the event that a lamp containing it should explode.
Accordingly, it is a fundamental object of the present invention to overcome this problem of extreme danger always present with the operation of a mercury arc vapor lamp by forming an inner sealed chamber for the lamp or illuminator within a double wall housing, while not compromising the performance of the illuminator.
It is believed that uneven or non-uniform heating and cooling over a typical period of operation of a mercury arc vapor lamp in the context already described may be the cause of the previously noted explosive, as well as other, failures.
Accordingly, it is a further object of the present invention to solve this underlying or causal problem by providing an efficient, integral cooling arrangement for the lamp to enable continuous dissipation of the heat generated by the lamp, thereby to avoid the undesirable non-uniform heating and cooling effects.
A very demanding requirement in the operation of a photolithographic pattern forming apparatus is that the apparatus must be continuously operative in order to produce the finished wafers on an uninterrupted, production line, basis.
Accordingly, it is yet another object of the invention to avoid an extended cooling down period for a failed mercury arc vapor lamp before trying to remove such lamp from its housing or assembly. Accordingly, the length of time that the stepper pattern forming apparatus is not available for production is minimized.